5 research outputs found
Mode Mixing and Substrate Induced Effect on the Plasmonic Properties of an Isolated Decahedral Gold Nanoparticle
We report new results on the localized
surface plasmon (LSP) assisted
optical effects of a single noble metal nanoparticle (MNP) in nm level
spectral and spatial domain which is related to the phase retardation
of electromagnetic signal with larger particle size. Site selective
electron beam excitation in a scanning electron microscope (SEM) show
multiple resonance peaks in the cathodoluminescence (CL) spectra of
an isolated gold decahedron of side edge length 230 nm sitting on
a silicon (Si) substrate. Apart from a substrate induced LSP mode
in the near-infrared (750 nm) region, finite-difference time-domain
(FDTD) numerical analysis also identifies two prominent LSP modes
in the visible region. While the shorter wavelength (560 nm) mode
has a mixture of in-plane quadrupolar and out-of-plane quadrupolar
charge distribution pattern, the longer wavelength (655 nm) mode has
the dipolar charge pattern in both the direction. We also analyze
numerically to show the critical size of the side edge length of the
decahedron particle where mode mixing is initiated
Annealing Induced Morphology of Silver Nanoparticles on Pyramidal Silicon Surface and Their Application to Surface-Enhanced Raman Scattering
This
paper reports on a simple and cost-effective process of developing
a stable surface-enhanced Raman scattering (SERS) substrate based
on silver (Ag) nanoparticles deposited on silicon (Si) surface. Durability
is an important issue for preparing SERS active substrate as silver
nanostructures are prone to rapid surface oxidation when exposed to
ambient conditions, which may result in the loss of the enhancement
capabilities in a short period of time. Here, we employ the galvanic
displacement method to produce Ag nanoparticles on Si(100) substrate
prepatterned with arrays of micropyramids by chemical etching, and
subsequently, separate pieces of such substrates were annealed in
oxygen and nitrogen environments at 550 °C. Interestingly, while
nitrogen-annealed Si substrates were featured by spherical-shaped
Ag particles, the oxygen annealed Si substrates were dominated by
the formation of triangular shape particles attached with the spherical
one. Remarkably, the oxygen-annealed substrate thus produced shows
very high SERS enhancement compared to the either unannealed or nitrogen
annealed substrate. The hitherto unobserved coexistence of triangular
morphology with the spherical one and the gap between the two (source
of efficient hot-spots) are the origin of enhanced SERS activity for
the oxygen-annealed Ag particle-covered Si substrate as probed by
the combined finite-difference time domain (FDTD) simulation and cathodoluminesensce
(CL) experiment. As the substrate has already been annealed in an
oxygen environment, further probability of oxidation is reduced in
the present synthesis protocol that paves the way for making a novel
long-lived thermally stable SERS substrate
Efficient Excitation of Higher Order Modes in the Plasmonic Response of Individual Concave Gold Nanocubes
Recently,
concave nanocube (CNC) shaped metal nanoparticles (MNPs)
with high index facets have drawn special attention due to their high
chemical activity and large electromagnetic (EM) field enhancements,
making them good candidates for multifunctional platforms. However,
most of the previously published works focused on the plasmonic properties
of silver simple nanocubes of smaller dimension, i.e., within the
quasi-static limit, hardly supporting efficient excitation of high-order
plasmonic modes. Site-selective electron beam excitation of individual
Au CNC particles gives rise to simultaneous excitation of edge and
corner localized surface plasmon (LSP) modes. We show that spatial
variation of the radiative modes is strongly localized at the corners
and extended along the edges of the top surface of the CNCs. Extensive
finite-difference time-domain (FDTD) numerical analysis reveals that
the substrate-induced plasmon hybridization leads to the activation
of corner octupolar and corner quadrupolar LSP modes, in agreement
with the cathodoluminescence (CL) measurements. Remarkably, the strength
of the hybridization is shown to depend on the CNC size. Furthermore,
we show that the edge quadrupolar mode becomes prominent with increasing
concaveness, thus opening up a new way of engineering the LSP modes
Effect of Intertip Coupling on the Plasmonic Behavior of Individual Multitipped Gold Nanoflower
We report here, the first experimental
realization on the selective
excitation of two closely lying tips from the same spherical core
of a multitipped gold nanoparticle with flower-like morphology. This
gives strong multipeaked resonance in the near-infrared region of
the far-field emission spectra showing a clear signature of tip to
tip coupling. The cathodoluminescence (CL) technique in a scanning
electron microscope (SEM) combined with finite-difference time-domain
(FDTD) simulation has helped us to identify the coupled plasmon modes
to be originated from the interaction between two closely spaced tips
with a narrow angular separation. Our analysis further estimates a
range of angular separation between the tips that triggers the onset
of the intertip coupling
Probing Localized Surface Plasmons of Trisoctahedral Gold Nanocrystals for Surface Enhanced Raman Scattering
Trisoctahedral (TOH)
shaped gold (Au) nanocrystals (NCs) have emerged
as a new class of metal nanoparticles (MNPs) due to their superior
catalytic and surface enhanced Raman scattering (SERS) activities
caused by the presence of high density of atomic steps and dangling
bonds on their high-index facets. We examine the radiative localized
surface plasmon resonance (LSPR) modes of an isolated single TOH Au
NC using cathodoluminescence (CL), with high resolution spatial information
on the local density of optical states (LDOS) across the visible spectral
range. Further, we show pronounced enhancement in the Raman scattering
by performing Raman spectroscopic measurements on Rhodamine 6G (R6G)-covered
TOH Au NPs aggregates on a Si substrate. We believe that the hot spots
between two adjacent MNP surfaces (“nanogaps”) can be
significantly stronger than single particle LSPRs. Such “nanogaps”
hot spots may have crucial role on the substantial SERS enhancement
observed in this report. Consequently, the present study indicates
that MNPs aggregates are highly desirable than individual plasmonic
nanoparticles for possible applications in SERS based biosensing